(1) Field of the Invention
The present invention relates to a fluorescent lamp and a high intensity discharge lamp.
(2) Related Art
Fluorescent lamps and high intensity discharge (HID) lamps are widely known to emit light with high efficiency.
A fluorescent lamp includes an arc tube in which mercury and a rare gas are enclosed. The inner surface of the arc tube is coated with phosphors. The electric discharge performed in the arc tube excites mercury to emit ultraviolet light with the dominant wavelength of 254 nm. The ultraviolet light excites the phosphors to emit visible light. In this way, a luminous flux can be obtained. Typical fluorescent lamps of this type have conventionally been straight tube type fluorescent lamps and circular fluorescent lamps, with bulb-type fluorescent lamps and compact fluorescent lamps being widely introduced in recent years.
An HID lamp is the generic name for a high-pressure mercury lamp, a metal halide lamp, and a high-pressure sodium lamp.
The high-pressure mercury lamp emits light due to the electric discharge under mercury vapor of 100 to 100 kPa.
The metal halide lamp emits light as follows. With the electric discharge, metal halide is dissociated into metallic atoms and halide atoms. The metallic atoms are then excited to emit visible light.
The high-pressure sodium lamp emits light due to the electric discharge under sodium vapor.
As basic performances of these fluorescent lamps and HID lamps, obtaining a larger luminous flux with lower electric power consumption and achieving a long lifetime are pursued. Active research and development have been made for accomplishing these basic performances.
As one example, Japanese Laid-Open Patent Application No. H11-167899 discloses a technique for lengthening a lifetime of a fluorescent lamp. According to the disclosure, the luminous intensity of a conventional fluorescent lamp employing soda glass is likely to decrease because sodium is eluted from the soda glass at the time the fluorescent lamp is manufactured or lit, and the eluted sodium reacts with mercury. In view of this, the fluorescent lamp according to the technique employs such glass from which sodium is less likely to be eluted than the conventional soda glass, for preventing the luminous intensity from decreasing.
Also, to obtain a larger luminous flux of a fluorescent lamp with lower electric power consumption, for example, research and development have been made to improve luminance of phosphors, and to secure a long arc length by making an arc tube thinner.
These research and development have contributed to improving the performances of fluorescent lamps and HID lamps to some extent. However, there are increasing demands for further improving these performances in recent years. To meet these demands, techniques for further decreasing the electric power consumption and providing larger luminous flux are called for.